Principles of Communicartion System by N Hasan

Principles of Communication Systems

Chapter 3 (part 2): FM Transmitter & Receiver

byNurulfadzilah Hasan

Faculty of Electrical & Electronics Engineeringnurulfadzilah@ump.edu.my

For updated version, please click on http://ocw.ump.edu.my

Principles of Communicartion System by N Hasan

Lesson Outcomes

• By the end of this topics students should be able to:– Explain how FM transmitters and receivers

operate– Describe how FM overcomes noise

Principles of Communicartion System by N Hasan

FM Transmitter

Principles of Communicartion System by N Hasan

Description

Principles of Communicartion System by N Hasan

FM Receiver

Principles of Communicartion System by N Hasan

Description (cont.)

Principles of Communicartion System by N Hasan

Discriminator

Discriminator (or slope detection) converts FM signal to AM signal and then by using AM demodulation circuit is to get back the information signal.

Principles of Communicartion System by N Hasan

Frequency modulators

Two common types of frequency modulator

Principles of Communicartion System by N Hasan

Varactor Modulator

Principles of Communicartion System by N Hasan

Reactance Modulator

• Reactance modulator works by using transistoramplifier as variable capacitor or an inductor.

• When the circuit is connected across the tunedcircuit of an oscillator, the oscillator frequencycan be varied by applying the modulating signalto the amplifier.

• Reactance modulators can produce frequencydeviation over a wide range.

• Reactance modulators are highly linear, sodistortion is minimal.

Principles of Communicartion System by N Hasan

A Reactance Modulator

Image source: https://et.wikipedia.org/wiki/Fail:Reaktiivmodulaator.png

Principles of Communicartion System by N Hasan

Frequency Demodulators

Slope Detector

Pulse-Averaging Discriminators

Quadrature Detector

Phase-Locked Loop Detector

Principles of Communicartion System by N Hasan

Slope Detector

• Slope detector uses tuned circuit and a diode detector toconvert frequency variations into voltage variations.

• The main difficulty with slope detectors is tuning them.

Principles of Communicartion System by N Hasan

Pulse-Averaging Discriminators

• Consists of a zero crossing detector, a one shot multi vibratorand a low-pass filter.

• A very high-quality frequency demodulator.• This discriminator is used in many electronic products.

Principles of Communicartion System by N Hasan

Quadrature Detector

• Most widely used FM demodulator.• Uses a phase-shift circuit to produce a phase shift of 90

degrees at the unmodulated carrier frequency.

Principles of Communicartion System by N Hasan

Phase-Locked Loops

• Phase-locked loop (PLL) is a frequency-sensitive feedbackcontrol circuit used in frequency demodulation

• PLLs have three basic elements: Phase detector, Low-passfilter, Voltage-controlled oscillator (VCO)

Principles of Communicartion System by N Hasan

Noise in FM

• Noise is interference that affect information signal.

• In FM, modulated signals have a constant amplitude.

• At receiver, limiter circuits clipped any amplitude variationsthat may exist on the received signal.

• Information content of the FM signal is not affected since it iscontained by the frequency variations, not amplitude of thereceived signal.

Principles of Communicartion System by N Hasan

Noise and Angle Modulation

• Noise distribution in FM is non-uniform. • Higher frequencies suffers more noise than the lower frequencies. • Higher frequencies also have lower amplitudes compared to lower

frequencies.• Consequently, the Signal to noise ratio is also non-uniform.• The SNR for higher-modulating frequencies are lower than the SNR for the

lower frequencies.

Without preemphasis

Principles of Communicartion System by N Hasan

• Solution: the high-frequency modulating signals are emphasized or boosted in amplitude of the transmitter prior to performing modulation and then deemphasized or attenuated during demodulation at receiver.

With preemphasis

Principles of Communicartion System by N Hasan

Principles of Communicartion System by N Hasan

Pre-emphasis

De-emphasis

Pre-emphasis & De-emphasis circuits

Principles of Communicartion System by N Hasan

FM Application

Radio broadcasting

Two way mobile radio

Microwave communication

TV sound transmission

Cellular radio communication

Satellite communication

Principles of Communicartion System by N Hasan

Collaborative authors:

Nurulfadzilah Binti Hasan Noor Zirwatul Ahlam Binti Naharuddin

Norhadzfizah Binti Mohd RadiMohd Hisyam Bin Mohd Ariff

Faculty of Electrical & Electronics Engineering, UMP